A document scanner is an apparatus that converts printed text into digital data by illuminating the text with a fluorescent lamp and applying optical character recognition methods to the text. A problem with prior art document scanners is that the light intensity produced by their fluorescent lamps varies as both a function of age and temperature. For example, when a fluorescent lamp is first energized, light output increases to a maximum value as the lamp warms up, but then decreases as the lamp temperature continues to rise, .until equilibrium is reached. Moreover, darkening of the ends of the fluorescent lamp bulb causes lumen output to decrease as the lamp ages (lamps generally exhibit a significant decrease in lumen output after about 100 hours of operation). Further, lumen output may change with changes in power supply output. The need to correct these problems has increased with the advent of color document scanners, since constant light intensity is necessary for accurate reproduction of color documents.
Therefore, an object of the present invention is to provide methods and apparatus for automatically maintaining the light output of a fluorescent lamp at a substantially uniform and controlled level. A further object of the present invention is to reduce end darkening and effects thereof, particularly in a document scanner.